The present invention relates to a process and apparatus for producing silicon ingots having a high oxygen content by crucible-free zone pulling, silicon ingots obtainable thereby and silicon wafers produced therefrom.
More particularly, the invention relates to such a process wherein a molten zone is produced between a feed ingot and a product ingot with the aid of a surrounding induction heating coil. Starting from the feed ingot, the molten zone passes through the annular hole of the coil in the form of a molten neck and then expands to form a molten cap covering the product ingot, the molten zone being brought into contact with quartz shaped parts during the pulling process.
In the processes for producing electronic components having high packing density from silicon wafers, the oxygen content of the silicon plays an ever more important role. The reason is that the intrinsic gettering action of the oxygen is ever more frequently exploited in certain process steps. In microelectronics, use is, therefore, almost exclusively made of silicon which has a minimum oxygen content of about 10.sup.16 atoms of oxygen/cm.sup.3. Such silicon is obtained by Czochralski crucible pulling, since oxygen is accumulated in the melt and incorporated in the growing crystal ingot in the course of the pulling operation as a result of the reaction between the silicon melt and the quartz crucible. In addition, the oxygen incorporated also has a certain hardening effect.
On the other hand, silicon obtained by crucible-free zone pulling has a markedly lower oxygen content of typically about 10.sup.15 to 10.sup.16 atoms of oxygen/cm.sup.3 of silicon owing to the lack of contact between the silicon melt and quartz crucible. Instead, it is distinguished, however, by higher purity and higher minority carrier lifetime compare with crucible-pulled material and also by the fact that particularly high resistances can be achieved. The main field of application of the zone material is, therefore, the sector covering the production of power components.
Hitherto, many attempts have been made to obtain silicon crystals having higher oxygen content by means of zone pulling and to combine, in this manner, the advantages of zone material with those of crucible material. For example, as described in DE-A-3,333,960 or the corresponding U.S. Pat. No. 4,722,764, a crucible-pulled polycrystalline feed ingot, which, therefore, has an enhanced oxygen content is used, instead of a feed ingot produced by gas-phase deposition of polycrystalline silicon. Owing to the crucible pulling step, the material obtained has, however, an enhanced impurity level and, in addition, loses some of the oxygen as a result of degassing during the zone pulling step.
In the process as claimed in EP-A-54,657, an oxygen atmosphere is established in the zone pulling apparatus. In that case, however, it is difficult to adjust the oxygen flow so as to achieve a constant oxygen content over the entire ingot length, especially as the variation in the convection flow in the container with the change in length of the feed ingot and the product ingot can result in defects.
EP-A-140,239 describes a process in which a quartz rod is presented to the molten neck surrounded by the coil and immersed in the melt at that point, in which process it gradually dissolves and consequently delivers oxygen, and possibly also other dopants, to the silicon. However, this process results in the incorporation of oxygen in the crucible material in suitable concentration ranges only in the case of ingots having small ingot diameters. Such high rates of incorporation can no longer be achieved, however, with this method in the case of larger ingot diameters from about 75 mm upwards having correspondingly larger free melt surfaces. In addition, there is the risk that silicon starts to crystallize out at the point of immersion of the rod, as a result of the heat removal. The crystallites formed detach themselves and float on the surface of the melt to the interface with the product ingot, where they can cause the formation of dislocations. The pulling process then has to be terminated, since only dislocation-free silicon single-crystal ingots are suitable for further processing to silicon wafers.